code: def test_retrieve_file_with_callback(self):
      func = mock.Mock()
      _buffer = StringIO("buffer")
      with fh.FTPHook() as ftp_hook:
          ftp_hook.retrieve_file(self.path, _buffer, callback=func)
      self.conn_mock.retrbinary.assert_called_once_with(
          "RETR /some/path", func, 8192
      )
  
imports: from __future__ import annotations
  from io import StringIO
  from unittest import mock
  import pytest
  from airflow.providers.ftp.hooks import ftp
label_desc: Use image node_id 14 for calling the TestFTPHook obj's underlying member method code with example usage: obj.test_retrieve_file_with_callback() without return types